Question

why the efficiency of a heat engine can't be 100%?

Answer 1

This is because a part of input energy is wasted in the form of heat. So , output can't be 100%

Answer 2

This heat rejected out makes yourengine less than 100 percent efficient! Basically, the very nature of heat means that some of the energy will escape into the surrounding area. ... The same concept applies to your question about the efficiency of a heatengine.